The present invention relates generally to coated stents and more particularly to a method for coating a superelastic self-expanding stent.
Stents are used to treat various organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like. For example, stents are commonly used to treat blockages, occlusions, narrowing ailments and other similar problems that restrict flow through a passageway. One area where stents are commonly used for treatment involves implanting an endovascular stent into the vascular system in order to improve or maintain blood flow through narrowed arteries. However, stents are also used in other treatments as well, such as the treatment of aneurysms. Stents have been shown to be useful in treating various vessels throughout the vascular system, including both coronary vessels and peripheral vessels (e.g., carotid, brachial, renal, iliac and femoral). In addition, stents have been used in other body vessels as well, such as the digestive tract.
Many different types of stents and stenting procedures are possible. In general, however, stents are typically designed as tubular support structures that may be inserted percutaneously and transluminally through a body passageway. Traditionally, stents are made from a metal or other synthetic material with a series of radial openings extending through the support structure of the stent to facilitate compression and expansion of the stent. Although stents may be made from many types of materials, including non-metallic materials, common examples of metallic materials that may be used to make stents include stainless steel, nitinol, cobalt-chrome alloys, amorphous metals, tantalum, platinum, gold and titanium. Typically, stents are implanted within a passageway by positioning the stent within the area to be treated and then expanding the stent from a compressed diameter to an expanded diameter. The ability of the stent to expand from a compressed diameter makes it possible to thread the stent to the area to be treated through various narrow body passageways while the stent is in the compressed diameter. Once the stent has been positioned and expanded at the area to be treated, the tubular support structure of the stent contacts and radially supports the inner wall of the passageway. As a result, the implanted stent mechanically prevents the passageway from narrowing and keeps the passageway open to facilitate fluid flow through the passageway.
Self-expanding stents are increasingly used and accepted by physicians for treating a variety of ailments. Self-expanding stents are usually made of shape memory materials or other elastic materials that act like a spring. Typical metals used in this type of stent include nitinol and 304 stainless steel. A common procedure for implanting a self-expanding stent involves a two-step process. First, the narrowed vessel portion to be treated is dilated with a balloon but without a stent mounted on the balloon. Second, a stent is implanted into the dilated vessel portion. To facilitate stent implantation, the stent is installed on the end of an inner catheter in a compressed, small diameter state and is usually retained in the small diameter by inserting the stent into a restraining sheath at the end of the catheter. The stent is then guided to the balloon-dilated portion and is released from the inner catheter by pulling the restraining sheath away from the stent. Once released from the restraining sheath, the stent radially springs outward to an expanded diameter until the stent contacts and presses against the vessel wall. Traditionally, self-expanding stents have been more commonly used in peripheral vessels than in coronary vessels due to the shape memory characteristic of the metals that are used in these stents. One advantage of self-expanding stents for peripheral vessels is that traumas from external sources (e.g., impacts to a person's arms, legs, etc.) which are transmitted through the body's tissues to the vessel do not permanently deform the stent. Instead, the stent may temporarily deform during an unusually harsh trauma but will spring back to its expanded state once the trauma is relieved.
One type of self-expanding stent that is commonly preferred is superelastic self-expanding stents. Superelastic self-expanding stents are usually made from nitinol and remain elastic throughout an unusually large range of deformation. Thus, a superelastic self-expanding stent can have an especially large expansion ratio, which allows the stent to be compressed down to a particularly small diameter for delivery to a treatment site and yet can elastically expand sufficiently to contact and exert pressure against a vessel wall when released. This is desirable to minimize trauma during the delivery process, and also to ensure that the stent exerts a desirable level of radial force against the vessel wall once implanted.
In order to provide improved treatment results at the treatment site, some stents are coated with a drug or other type of coating. For example, a stent may be coated with a drug like paclitaxel, which is an antiproliferative that prevents renarrowing of a vessel wall. Stents may also be coated with heparin, which is an anticoagulant that prevents blood clotting. However, these are only a few examples of the types of coatings that can be applied to a stent. One problem with the use of coatings on superelastic self-expanding stents is that the coating can be damaged when the stent is compressed and loaded into the delivery system. This may be a particular problem when the coating is a drug coating since uniform distribution of the drug is typically desirable for consistent treatment results.
Accordingly, the inventors believe it would be desirable to provide an improved method for coating superelastic self-expanding stents.